Temporal and geochemical evolution of the Guichon Creek Batholith and Highland Valley porphyry copper district, British Columbia: implications for generation and tectonic setting of porphyry systems

Abstract

Guichon Creek Batholith, a Late Triassic, calc-alkaline composite batholith of the Nicola arc, British Columbia, hosts major porphyry copper deposits of the Highland Valley district. U-Pb zircon and Ar-Ar age dating of the main intrusive phases of this normally compositionally zoned batholith indicate that it evolved over a 7 M.y. period (215.6 ± 0.5 to 208.6 ± 0.6 Ma). The batholith consists of two distinct geochemical suites, the barren pre-211 Ma Highland Valley, and the mineralized post-211 Ma Bethlehem suite. Ages of host intrusion and a cross-cutting, post-mineralization dyke date the Bethlehem deposit to within a <1 M.y. period at ~210 Ma. This event predates the Bethsaida pluton, host to Valley, Lornex and Highmount deposits. These younger deposits formed after 209.1 ± 0.3 Ma, prior to or synchronously with the Gnawed Mountain porphyry at 208.6 ± 0.6 Ma. Ar-Ar and U-Pb ages are generally similar, indicating short-lived and rapidly cooled magmatic/hydrothermal systems. Published Re-Os molybdenite ages at Valley suggest a third mineralizing event at 206.5 Ma. Mineralization is linked to the change in magma composition between the Highland Valley and Bethlehem suites, both of which have distinctive high Sr/Y, La/Yb and Gd/Yb signatures. The Bethlehem suite is more calcic, hydrous and oxidized than Highland Valley suite. Both suites were likely derived by melting hydrous, metal-enriched mafic lower crustal cumulates residual from earlier arc magmatism. Partial melting of such protoliths is thought to have been triggered by arc-arc collision, followed by slab detachment and upwelling of hot asthenospheric mantle-derived mafic magmas. Termination of Triassic arc calc-alkaline volcanism overlapped with the ca 216 Ma outer mafic Border phase. Emplacement (211-208 Ma) of batholith interior phases was accompanied by porphyry Cu formation at 210 and ~208 Ma. Based on this study, the fertile Guichon Creek Batholith represents a template for understanding porphyry deposit formation and distribution that can facilitate exploration targeting.